ASE G1 Auto Maintenance and Light RepairAutomatic Transmission/TransaxleMedium
An automatic transmission is exhibiting erratic shifting patterns, sometimes shifting too early, sometimes too late, and occasionally feeling harsh. The fluid level and condition are good. A scan tool reveals no Diagnostic Trouble Codes (DTCs). Which of the following external controls is most likely contributing to these symptoms?
- AA misadjusted manual linkage.
- BA worn internal detent spring.
- CA clogged transmission cooler.
- DA faulty vehicle speed sensor (VSS).
Show answer & explanationAnswer & explanation
Correct answer: D. A faulty vehicle speed sensor (VSS).
The vehicle speed sensor (VSS) provides crucial input to the Transmission Control Module (TCM) for determining shift points. An erratic VSS signal would directly lead to the TCM making incorrect shift decisions, resulting in erratic, early, late, or harsh shifts without necessarily setting a specific DTC if the signal loss is intermittent or plausible-but-wrong.
Why the other options are wrong
- A. A misadjusted manual linkage typically affects gear selection (e.g., P/N alignment) or prevents certain gears, not erratic shifting patterns across the board.
- B. A worn internal detent spring affects the feel of the shifter handle, not the actual automatic shifting patterns of the transmission.
- C. A clogged transmission cooler would cause overheating and fluid breakdown, leading to slipping or harsh shifts, but not typically erratic, inconsistent shift points without other fluid issues.
Vehicle Speed Sensor (VSS)
The Vehicle Speed Sensor (VSS) measures the rotational speed of the transmission output shaft or wheel, providing critical data to the PCM/TCM for shift scheduling, speedometer operation, and cruise control.
- Essential for accurate shift points.
- Faulty VSS can cause erratic shifting or speedometer malfunction.
- Signal typically sent to PCM/TCM via a reluctor wheel.
Memory trick: Speed, Throttle, RPM, and Temperature Guide the Shifts.